This invention relates to fire resistant coatings and to cellulosic materials modified with such coatings to render them fire resistant. The invention relates also to methods of making cellulosic materials fire resistant.
Cellulosic materials such as timber and particle board are widely used as building materials. There are many cellulosic materials for which it is desirable to have some degree of fire resistance. For example, there are many building materials which, if rendered at least with some degree of fire resistance, will slow down the process of a fire contained therein thus providing essential safety for any occupants thereof. These building materials include timber, wall boards of many types; surface materials that are placed within a building structure, for example, ceiling tiles. If these materials could be rendered at least partially fire resistant, any resulting fire would be of a slow burning nature and thus improve the safety of the occupants of the building.
Cardboard is a cellulosic material that generally has a limited use as a building material being usually restricted to use as reinforcing in flush panel doors. However, cardboard offers the desirable properties of relatively high strength to weight ratio, especially stiffness, combined with low cost. In the case of corrugated cardboard it also offers low thermal conduction which is attributed to the presence of air entrapped in the corrugated cardboard flutes. Despite these attractive benefits, the use of cardboard has been limited because of its combustibility and lack of adequate fire resistance. By fire resistance it is meant that cellulosic materials treated as taught within the ambit of this invention by the solutions described herein, will exhibit a substantial reduction in the propensity to support a fire.
There have been various attempts to improve the fire resistance of cellulosic materials. In the case of cardboard material, these have included soaking cardboard panels in aqueous solution of sodium silicate. However, such coatings have not proven satisfactory as they have poor water resistance. Furthermore, on exposure to flames the coating can melt or crack exposing the combustible cardboard to the flames.
Dimanshtaeyn, U.S. Pat. No. 5,035,851, Jul. 30, 1991 describes the use of coating solution which includes a silicate, a clay and some inorganic materials (e.g. a borate) which can be used to coat metals, woods and foamed polymeric materials to impart some degree of fire resistance thereto. This is a complicated and expensive solution and acceptable resistance to fire is not always achieved.
Luckanuck in U.S. Pat. No. 5,085,897, Feb. 4, 1992 describes a liquid mixture of silicate and an inert mineral fibre a mineral powder which is used to coat steel beams used within buildings. This solution, when coated on the steel building materials, is said to help reduce twisting of steel columns and other building materials in a fire. The treatment does not include a reactive calcined filler nor a latent acid catalyst.
Nguyen et al., U.S. Pat. No. 4,888,057, Dec. 19, 1989 describes a composite fire resistant coating which comprises a mixture of silicates and silicon carbide powder. It is said that building materials coated with these materials are resistant to fire. However, this coating is complicated and expensive to use.
U.S. Pat. No. 6,040,057 (Mar. 21, 2000) involves the treatment of a cellulosic substrate with an alkali metal treated material to render the alkali metal silicate water insoluble.
U.S. Pat. No. 3,940,516 (Feb. 24, 1976) relates to compositions consisting of:
5 to 15% of powdered silicate-oxide sinter or alloy
15 to 35% of an anhydrous powdered alumina silicate
50 to 80% of acid phosphates of aluminium, chromium, magnesium or calcium.
The above compositions lack alkali metal silicates.
U.S. Pat. No. 5,171,496 (Dec. 15, 1992) discloses wood composites. The compositions include blast furnace slag and are cured by heating. The compositions are not ordinarily fire resistant but can be rendered such by modifying the cellulosic materials. The composition without the treated cellulose is thus not a fire resistant composition. These compositions of this citation are also calcium rich and this is important to this reaction mode.
This invention provides in one form an aqueous fire resistance treatment composition comprising:
water;
a metal silicate selected from an alkali metal silicate or an alkaline earth metal silicate;
a reactive calcined filler;
a latent acid catalyst.
The alkali metal silicate is preferably selected from the group consisting of potassium silicate, sodium silicate and lithium silicate. The alkaline earth metal silicate solution is preferably selected from the group consisting of beryllium silicate, magnesium silicate and calcium silicate.
Preferably the metal silicate is selected from the group consisting of sodium silicate and potassium silicate.
More preferably the metal silicate is sodium silicate.
Preferably the reactive calcined filler is selected from the group consisting of alumina and alumino silicates.
In an alternative form this invention provides a method of treating cellulosic substrates to render them fire resistant by treating the substrate with a composition comprising:
water;
a metal silicate selected from an alkali metal silicate or an alkaline earth metal silicate;
a reactive calcined filler;
a latent acid catalyst
and allowing the treatment to dry and cure.
Preferably the treatment includes a dried, cured coating having a film thickness in the range 50-1000 xcexcm.
In a still further form this invention provides cellulosic material treated with a fire resistant composition formed by curing a composition comprising:
water;
a metal silicate selected from an alkali metal silicate or an alkaline earth metal silicate;
a reactive calcined filler;
a latent acid catalyst.
Preferably the cellulosic material is coated with the fire resistant composition.
Sodium silicate is the preferred metal silicate. However, other metal silicates may be used including mixtures of different metal silicates.
The preferred sodium silicate is manufactured by PQ Industries. This material is an aqueous solution which is basically comprised of SiO2/Na2O.
Examples of suitable solutions of sodium silicate are Vitrosol N and Vitrosol H, both available from PQ Australia. Vitrosol N is a 38% w/w solution in water.
The reactive calcined filler is preferably selected from calcined alumina and calcined alumino silicate. These materials fall into the class of fillers known as pozzolans. These are defined as materials which in finely divided form and in the presence of water, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties. Examples of suitable materials are calcined flint clay, calcined alumina, fly-ash and blast furnace slag. The filler is described as reactive in that it can react with alkaline water and/or the metal silicate. The filler is thus distinct from conventional fillers such as talc and clay. These reactive fillers are readily available and generally of low cost. Fly-ash is a finely divided glossy material generated from combustion of pulverised coal in modern power plants. They have previously found use in modified concretes where lower costs and higher long term strengths can be achieved. The particle size of the reactive fillers is important for best results and these are achieved when the maximum particle size is less than 150 xcexcm. Generally better results are achieved with smaller particles and those that pass through a 75 xcexcm sieve produce improved performance.
The latent acid catalyst is preferably a modified organic acid, especially an ester, which becomes active under the conditions of treatment with the composition. Preferred latent acid catalysts are esters of acetic acid and esters of dibasic acids such as glutaric, succinic and adipic. An example of suitable latent acid catalysts is glycerol triacetate. Under the alkaline conditions of the composition the ester group hydrolyses leaving free acid which acts as the catalyst. Various other substances, such as phosphates and borates, will also hydrolyse in the aqueous, alkaline mixture of the invention, in the process reducing the pH and causing the mixture to set. An example of such latent catalysts is trisodium meta phosphate. The use of latent acid catalysts allows adequate pot-life to allow the treatment composition to be applied onto the surface. The selection of latent acid catalyst allows the reaction time to be adjusted to give adequate pot-life for the method of application. Generally, the dibasic acid catalysts give slower set and longer pot-life than the glycerol triacetate. Latent catalysts are materials which do not act as catalysts themselves or are relatively inactive but are converted to catalysts or more active catalysts by means of chemical or physical changes. It should be noted that, in the context of this patent, a latent catalyst includes any substance that, when added to the alkaline mixture of the invention, chemically or physically changes to a substance that reduces the pH of the mixture, causing the mixture to set. Such substances are preferably added in anhydrous form, in which case they often absorb excess moisture in the formulation, thus speeding the drying of the finished produce A further example of a latent catalyst is the use of carbon dioxide, preferably present in the atmosphere, or supplied via a pressure vessel. The carbon dioxide can form carbonic acid on absorption into the composition thereby causing the composition to set. A further example of a latent acid catalyst is one that becomes activated by increasing the temperature of the composition.
The relative properties of the constituents of the compositions of the present invention influence the properties of both the liquid and cured coatings. To enable application by dipping or flow coating it is important that the composition have relatively low viscosity. Viscosities in the range of 150-250 cps have been found to be particularly suitable. However, viscosities considerably in excess of 250 cps can be used. For example, viscosities of 2500 cps or more may be suitable, especially if the composition is shear thinning or pseudoplastic. Shear thinning rheology is well known and is characterised by a reduction in viscosity as the shear rate increases. The viscosity of the sodium silicate solution as modified with the filler is often suitable without further adjustment. However, additional rheology modifying agents, both organic and inorganic may be incorporated to adjust the rheology.
The weight ratio of reactive calcined filler to the metal silicate is normally in the range of 4:1 to 1:4, preferably 3:1 to 1:3 and more preferably 2:1 to 1:2. However, low levels of reactive filler can produce useful compositions. These weight ratios are expressed as the weight of reactive calcined filler to the metal silicate as a 38% aqueous solution. When expressed on a non-volatile basis the range 4:1 to 1:4 becomes 10:1 to 1:1.5, 3:1 to 1:3 becomes 8:1 to 1:1.1, and 2:1 to 1:2 becomes 5:1 to 1:0.8. Relatively high levels of calcined filler can lead to thixotropic compositions which can be difficult to use with corrugated cardboard as the cellulosic substrate because the penetration of the composition into the flutes is poor. The cure rate at high levels of calcined filler is usually slower and this can be less useful in some circumstances. The quantity of latent acid catalyst is normally in the range of 1.0-10% of the total composition.
As well as the components specified above, other ingredients may also be present. These include surfactants or wetting agents which may effect the surface tension and surface wetting characteristics of the composition as well as the rheology of the composition. Preferred surfactants are non-ionic in nature and especially preferred are alkyl glucosides. The surfactants and wetting agents can also aid the dispersion of the filler.
Inert fillers such as clays and talcs may be used to modify the properties of the coatings. Other types of fillers may also be used to confer property enhancement or cost reduction. For example, hollow, glass or ceramic microspheres may be used to enhance thermal and other insulation properties. Anhydrous hygroscopic fillers may also be added. An example of such a material is anhydrous silica gel, used in desiccators. A further example is anhydrous sodium sulphate. Dyes and pigments may also be used. The use of these colouring materials can provide useful guides as to whether the substrate has been treated.
The compositions of the present invention are applied to cellulosic material substrates and the compositions can impregnate and/or form a coating on the substrate. The compositions can then be cured to form fire resistant compositions. The compositions may be applied by any of the usual methods for applying liquid compositions and include spray (air and airless), roller and dip coating. The selection of the most suitable application method will take into account the shape of the article. For substrates formed from corrugated cardboard, conventional coating techniques are quite suitable. However, these substrates may be treated before they are assembled. Reinforced cardboard walls are usually made from flat sheets adhered to interleaved corrugated sheets.
The invention will be described by reference to preferred embodiments described in the following examples where parts are expressed as parts by weight.